Sensor calibration device, and sensor calibration method

ABSTRACT

A sensor calibration ECU includes: a calibratability determination portion that determines whether or not a distance image sensor is in a calibratable state on the basis of at least one of the state of a vehicle and the state of a road surface on which the vehicle is positioned; and a calibration execution portion that calibrates the distance image sensor on the basis of a pre-found distance (=reference distance) between the distance image sensor and the road surface on which the vehicle is positioned, when it is determined by the calibratability determination portion that the distance image sensor is in the calibratable state.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2008-331373 filed onDec. 25, 2008 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a sensor calibration device and a sensorcalibration method for calibrating, for example, a distance-measuringsensor that is mounted in a vehicle. In particular, the inventionrelates to a sensor calibration device and a sensor calibration methodfor calibrating a distance image sensor that is mounted in a vehicle.

2. Description of the Related Art

A TOF (time of flight) method that calculates the distance to ameasurement object by measuring the time required for the round trip oflight to the measurement object is widely known as a distancemeasurement method. Besides, there is developed a distance image sensorthat obtains distance information by the TOF method by processingphotoelectric current prior to the voltage conversion of a lightreception signal, via a photogate that has a common CCD (charge coupleddevice) structure.

For example, an optical distance measurement device that obtains anaccumulated differential signal from an Ach signal from a firstaccumulation element and a Bch signal from a second accumulation elementby a differential computation portion is disclosed (see Japanese PatentApplication Publication No. 2007-132848 (JP-A-2007-132848)). Accordingto the optical distance measurement device described in Japanese PatentApplication Publication No. 2007-132848 (JP-A-2007-132848), since thedifferential computation with the Ach signal and the Bch signal isperformed, noise components, such as background light and the like, canbe appropriately removed, and only a signal component that is needed forthe calculation of the distance to the measurement object can beextracted and accumulated. Therefore, the device is able to performhighly accurate computation of distance in an environment with strongbackground light.

However, in the optical distance measurement device described inJapanese Patent Application Publication No. 2007-132848(JP-A-2007-132848), since the photoelectric conversion element, such asa CCD or the like, that accumulates charges that correspond to theamount of light received changes its characteristics depending on theoutside air temperature or the like, it is necessary to frequentlycalibrate the photoelectric conversion element in order to maintain gooddetection accuracy.

SUMMARY OF THE INVENTION

The invention provides a sensor calibration device and a sensorcalibration method that are capable of properly and easily calibrating adistance-measuring sensor such as a distance image sensor or the like.

A first aspect of the invention is a sensor calibration device thatcalibrates a distance-measuring sensor that is mounted in a vehicle, andincludes a calibratability determination portion, and a calibrationexecution portion. The calibratability determination portion determineswhether or not the distance-measuring sensor is in a calibratable statebased on at least one of state of the vehicle and state of a roadsurface on which the vehicle is positioned. The calibration executionportion calibrates the distance-measuring sensor based on a pre-founddistance between the distance-measuring sensor and the road surface onwhich the vehicle is positioned, when it is determined by thecalibratability determination portion that the distance-measuring sensoris in the calibratable state.

That is, since it is determined whether or not the distance-measuringsensor is in the calibratable state on the basis of at least one of thestate of the vehicle and the state of the road surface on which thevehicle is positioned, it can be properly determined whether or not thedistance-measuring sensor is in the calibratable state. Furthermore, thedistance-measuring sensor is calibrated on the basis of the pre-founddistance between the distance-measuring sensor and the road surface onwhich the vehicle is positioned. Therefore, the distance-measuringsensor, such as a distance image sensor or the like, can be properly andeasily calibrated.

The calibratability determination portion may determine whether or notthe distance-measuring sensor is in the calibratable state according towhether or not the vehicle is substantially parallel to the roadsurface.

With this construction, it can be properly determined whether or not thedistance-measuring sensor is in the calibratable state.

That is, in the case where the vehicle is substantially parallel to theroad surface, the distance-measuring sensor can be calibrated by usingthe pre-found distance between the distance-measuring sensor and theroad surface that has been found on the supposition that the vehicle isparallel to a flat road surface.

The distance-measuring sensor may be a distance image sensor at least aportion of whose detection region is the road surface, and thecalibratability determination portion may determine whether or not thedistance image sensor is in the calibratable state based on a result ofdetection by the distance image sensor.

With this construction, since it is determined whether or not thedistance image sensor is in the calibratable state on the basis of aresult of detection by the distance image sensor, there is no need todispose a sensor other than the distance image sensor. Therefore, thedistance-measuring sensor (the distance image sensor, in this case) canbe more easily calibrated.

The calibratability determination portion may determine whether or notthe vehicle is substantially parallel to the road surface based on theresult of detection by the distance image sensor, and then may determinewhether or not the distance image sensor is in the calibratable stateaccording to a result of determination as to whether or not the vehicleis substantially parallel to the road surface.

With this construction, in the case where the vehicle is substantiallyparallel to the road surface, the distance-measuring sensor can becalibrated by using the pre-found distance between thedistance-measuring sensor and the road surface that has been determinedon the supposition that the vehicle is parallel to a flat road surface.Besides, if a result of the detection by the distance image sensor isutilized, it is possible to properly determine whether or not thevehicle is substantially parallel to the road surface. For example, inthe case where the amount of change in the distance detected by each ofthe photosensitive cells contained in the distance image sensor is lessthan or equal to the pre-set threshold value, it can be determined thatthe vehicle is substantially parallel to the road surface.

The calibratability determination portion may determine that the vehicleis substantially parallel to the road surface when an amount of changein distance detected by each of photosensitive cells contained in thedistance image sensor during a pre-set time is less than or equal to apre-set threshold value.

With this construction, in the case where the amount of change in thedistance detected by each one of the photosensitive cells contained inthe distance image sensor during the pre-set time is “0”, it means thatthe vehicle is moving parallel to the road surface. Therefore, in thecase where the amount of change is less than or equal to a pre-setthreshold value, it can be determined that the vehicle is substantiallyparallel to the road surface.

The calibratability determination portion may determine that the vehicleis substantially parallel to the road surface when, regarding aplurality of pre-set photosensitive cells among photosensitive cellscontained in the distance image sensor, an amount of change in distancedetected by each of the pre-set photosensitive cells during a pre-settime is less than or equal to a pre-set threshold value.

With this construction, since it suffices that it is determined whetheror not the amount of change in the distance detected by each of thepre-set photosensitive cells during the pre-set time is less than orequal to the pre-set threshold value, the process that needs to beperformed in order to determine whether or not the vehicle issubstantially parallel to the road surface is simplified.

The calibratability determination portion may find a variance ofbrightnesses detected by photosensitive cells that are contained in thedistance image sensor, and may determine whether or not the distanceimage sensor is in the calibratable state according to whether or notthe variance is less than or equal to a pre-set threshold value.

With this construction, it can be properly determined whether or not thedistance image sensor is in the calibratable state.

Specifically, the higher the brightness of reflected light ofillumination, the higher the S/N ratio of the distance image sensor.Besides, in the case where a road surface has a water puddle or the likeand therefore the reflectance of the road surface is low, the brightnessdetected by each of the photosensitive cells of the distance imagesensor becomes low, and the S/N ratio of the distance image sensor alsobecomes low. Therefore, when the variance of the brightnesses of a roadsurface detected by the photosensitive cells contained in the distanceimage sensor is small, the road surface has a substantially uniformdistribution of brightness, so that it can be determined that thedistance image sensor is in the calibratable state.

Each of photosensitive cells contained in the distance image sensor maybe set in a corresponding one of a plurality of pre-set divided regions.The calibratability determination portion, with regard to each of thedivided regions, may find a variance of brightnesses detected by thephotosensitive cells contained in a divided region, and may determinewhether or not the divided region of the distance image sensor is in thecalibratable state according to whether or not the variance is less thanor equal to a pre-set threshold value. Furthermore, the calibrationexecution portion may calibrate the divided region of the distance imagesensor that is determined as being in the calibratable state by thecalibratability determination portion.

With this construction, among the plurality of divided regions, adivided region in which the road surface has a substantially uniformdistribution of brightness is determined as being in the calibratablestate. Therefore, it can be properly determined whether or not a dividedregion is in the calibratable state, with regard to each of the dividedregions. Besides, even in the case where not all the photosensitivecells contained in the distance image sensor can be calibrated (i.e.,the case where the distance image sensor as a whole is not in thecalibratable state), the calibration of the distance image sensor can beperformed with regard to each divided region, so that the distance imagesensor can be efficiently calibrated.

The sensor calibration device may further include a correctioncoefficient storage portion that stores a correction coefficient for usefor correcting a detected value from the distance image sensor, withregard to each of photosensitive cells contained in the distance imagesensor. Furthermore, the calibration execution portion may calibrate thedistance image sensor with regard to a plurality of brightnesses, andmay record in the correction coefficient storage portion a correctioncoefficient found as a result of calibration so that the correctioncoefficient is associated in correspondence with information about thebrightness in the correction coefficient storage.

With this construction, in the correction coefficient storage portion,the correction coefficients for correcting the detected values from thedistance image sensor are stored with regard to each of thephotosensitive cells contained in the distance image sensor.Furthermore, the calibration is performed for each of brightnesses, andthe correction coefficients found as results of the calibration arerecorded in the correction coefficient storage portion so that thecorrection coefficients correspond to the information about brightness.Therefore, proper calibration can be performed.

Specifically, since the calibration is performed for each of a pluralityof brightnesses and the correction coefficients found as results of thecalibration are stored in correspondence with the information aboutbrightness in the correction coefficient storage portion, propercorrection coefficients according to the brightness at the time ofdetection can be found from the correction coefficients that correspondto the brightnesses occurring at the time of the calibration. Forexample, in the case where two correction coefficients corresponding totwo brightnesses occurring at the time of calibration are stored in thecorrection coefficient storage portion, the two correction coefficientscan be linearly interpolated according to the brightness occurring atthe time of detection, so that proper correction coefficients can befound.

The sensor calibration device may further include a state detectionportion that detects state of the vehicle. The calibratabilitydetermination portion may determine whether or not thedistance-measuring sensor is in the calibratable state based on a resultof detection by the state detection portion.

With this construction, the state of the vehicle is detected, and it isdetermined whether or not the distance-measuring sensor is in thecalibratable state on the basis of a result of the detection of thestate of the vehicle. Therefore, it can be properly determined whetheror not the distance-measuring sensor is in the calibratable state.

The state detection portion may include at least one of an accelerationsensor that detects acceleration of the vehicle, an inclinationdetection sensor that detects inclination of the vehicle, and a vehiclespeed sensor that detects vehicle speed of the vehicle.

With this construction, the state of the vehicle is detected by at leastone of an acceleration sensor that detects the acceleration of thevehicle, an inclination detection sensor that detects the inclination ofthe vehicle, and a vehicle speed sensor that detects the vehicle speedof the vehicle. Therefore, it can also be properly determined whether ornot the distance-measuring sensor is in the calibratable state.

Specifically, in the case where the acceleration detected by theacceleration sensor is small, it can be estimated that the travelingstate of the vehicle is stable, and therefore it can be determined thatthe distance-measuring sensor is in the calibratable state. In the casewhere the inclination of the vehicle detected by the inclinationdetection sensor is small, it can be estimated that the vehicle issubstantially parallel to the road surface, and therefore it can bedetermined that the distance-measuring sensor is in the calibratablestate. Furthermore, in the case where the vehicle speed detected by thevehicle speed sensor is small, it can be estimated that the change inthe traveling state of the vehicle is small, and therefore it can bedetermined that the distance-measuring sensor is in the calibratablestate.

The inclination detection sensor may detect at least one of yaw angle,pitch angle and roll angle of the vehicle.

With this construction, it can also be properly determined whether ornot the distance-measuring sensor is in the calibratable state.

A second aspect of the invention is a sensor calibration method thatcalibrates a distance-measuring sensor that is mounted in a vehicle, andexecutes a calibratability determination step, and a calibrationexecution step. In the calibratability determination step, it isdetermined whether or not the distance-measuring sensor is in acalibratable state based on at least one of state of the vehicle andstate of a road surface on which the vehicle is positioned. In thecalibration execution step, the distance-measuring sensor is calibratedbased on a pre-found distance between the distance-measuring sensor andthe road surface on which the vehicle is positioned, when it isdetermined in the calibratability determination step that thedistance-measuring sensor is in the calibratable state.

Since it is determined whether or not the distance-measuring sensor isin a calibratable state on the basis of at least one of the state of thevehicle and the state of the road surface on which the vehicle ispositioned, it can be properly determined whether or not thedistance-measuring sensor is in the calibratable state. Furthermore,since the distance-measuring sensor is calibrated on the basis of thepre-found distance between the distance-measuring sensor and the roadsurface on which the vehicle is positioned, the sensor can be easilycalibrated.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, advantages, and technical and industrial significance ofthis invention will be described in the following detailed descriptionof example embodiments of the invention with reference to theaccompanying drawings, in which like numerals denote like elements, andwherein:

FIG. 1 is a block diagram showing an example of a construction of asensor calibration device in accordance with the invention;

FIG. 2A and FIG. 2B are illustrative diagrams showing examples of adetection region of a distance image sensor;

FIGS. 3A to 3C are illustrative diagrams showing examples of adistribution of brightnesses on a road surface SR which are detected bya CCD sensor of the distance image sensor;

FIG. 4 is a side view illustrating an example of a reference distancethat serves as a reference in the calibration;

FIG. 5A and FIG. 5B are graphs showing examples of a calculation methodfor correction coefficients which is performed by a calibrationexecution portion;

FIG. 6 is a table showing examples of correction coefficients stored ina correction coefficient storage portion;

FIG. 7A and FIG. 7B are graphs showing examples of a method ofcorrecting a detection value from the distance image sensor by usingcorrection coefficients stored in the correction coefficient storageportion;

FIG. 8 is a flowchart showing an example of an operation of a sensorcalibration ECU; and

FIG. 9 is a detailed flowchart showing an example of a calibratabilitydetermination process that is executed in step S101 and step S111 in theflowchart shown in FIG. 8.

DETAILED DESCRIPTION OF EMBODIMENTS

A sensor calibration device in an embodiment of the invention is adevice that calibrates a distance-measuring sensor that is mounted in avehicle. FIG. 1 is a block diagram showing an example of a constructionof such a sensor calibration device. As shown in FIG. 1, a sensorcalibration ECU (Electronic Control Unit) 1 (that can be regarded as aportion of a sensor calibration device) is connected to a distance imagesensor 2 and to an input appliance 3 so that the sensor calibration ECU1 can communicate therewith.

The distance image sensor 2 (that can be regarded as adistance-measuring sensor) includes an LED (Light-Emitting Diode) 21 anda CCD (Charge Coupled Device) sensor 22. The LED 21 projects light in apre-set direction (a rearward and obliquely downward direction, as canbe seen in FIGS. 2A and 2B). The CCD sensor 22 receives reflected lightthat is projected from the LED 21 and then is reflected by a roadsurface, or by an object such as a vehicle or the like. The distanceimage sensor 2 finds the distance to a road surface, or to an object,such as a vehicle or the like, by the TOF method, on the basis of theperiod from a time point of light emission from the LED 21 to a timepoint of reception of reflected light by the CCD sensor 22.

FIG. 2A and FIG. 2B are illustrative diagrams showing an example of adetection region of the distance image sensor 2. FIG. 2A is a side view,and FIG. 2B is a plane view. As shown in FIGS. 2A and 2B, the distanceimage sensor 2 is mounted in a rear end portion of a vehicle VC, and isdirected in a rearward and obliquely downward direction from the vehicleVC. Therefore, the detection region AR of the distance image sensor 2 isa quadrilateral pyramid-shape region that expands radially about animaginary line that extends in a rearward and obliquely downwarddirection from the distance image sensor 2. The detection region ARherein refers to a region in which distance can be detected by thedistance image sensor 2. Specifically, the distance to a road surfaceSR, or an object, such as a vehicle or the like, that exists within thedetection region AR is detected by the distance image sensor 2. Besides,in the case where no object exists within the detection region AR asshown in FIGS. 2A and 2B, most of the photosensitive cells (hereinafter,referred to simply as “cells”) that are provided within the CCD sensor22 of the distance image sensor 2 detect reflected light from the roadsurface SR, so that the distance to the road surface SR is detected.

Although this embodiment will be described in conjunction with the casewhere the distance-measuring sensor is the distance image sensor 2, theinvention is also applicable to a construction in which thedistance-measuring sensor is a different kind of distance-measuringsensor. For example, the distance-measuring sensor may also be a radarsensor, an ultrasonic sensor, etc.

Referring back to FIG. 1, the input appliance 3 of the sensorcalibration ECU 1 will be described. The input appliance 3 (which can beregarded as a portion of state detection means) includes an accelerationsensor 31, a yaw rate sensor 32, and a vehicle speed sensor 33. Theacceleration sensor 31 is a three-axis acceleration sensor that detectsthe acceleration of the vehicle VC in the up-down, left-right, andfront-rear directions, and outputs signals showing the accelerationsdetected in the up-down, left-right and front-rear directions to thesensor calibration ECU 1 (which, in this embodiment, is a statedetection portion 11).

Although this embodiment will be described in conjunction with the casewhere the acceleration sensor 31 detects the accelerations of thevehicle VC in the up-down, left-right and front-rear directions of thevehicle VC, the invention is also applicable to a construction in whichthe acceleration sensor 31 detects the acceleration of the vehicle VC inat least one of the up-down, left-right and front-rear directions of thevehicle VC.

The yaw rate sensor 32 (that can be regarded as an inclination detectionsensor) is made up of a rate integrating gyroscope or the like, anddetects a yaw rate that indicates a rate of change in the yaw angle(rotational angular speed about a vertical axis that passes through thecenter of gravity of the vehicle). The yaw rate sensor 32 outputs asignal showing the yaw angle, to the sensor calibration ECU 1 (in thisembodiment, the state detection portion 11). The vehicle speed sensor 33detects the vehicle speed of the vehicle VC, and outputs a signalshowing the vehicle speed to the sensor calibration ECU 1 (in thisembodiment, the state detection portion 11).

Although the embodiment will be described in conjunction with the casewhere the input appliance 3 includes the acceleration sensor 31, the yawrate sensor 32, and the vehicle speed sensor 33, the invention is alsoapplicable to a construction in which the input appliance 3 includes atleast one of the acceleration sensor 31, the yaw rate sensor 32, and thevehicle speed sensor 33.

Besides, although the embodiment will be described in conjunction thecase where the input appliance 3 includes the yaw rate sensor 32, theinvention is also applicable to a construction in which the inputappliance 3 includes an inclination detection sensor that detects theinclination of the vehicle VC. That is, the input appliance 3 mayinclude an inclination detection sensor that detects at least one of thepitch angle and the roll angle, instead of (or in addition to) the yawrate sensor 32.

Besides, the sensor calibration ECU 1 includes the state detectionportion 11, a calibratability determination portion 12, a calibrationexecution portion 13, and a correction coefficient storage portion 14,in terms of function. Incidentally, the sensor calibration ECU 1 causesa microcomputer that is disposed at an appropriate location in thesensor calibration ECU 1 to execute control programs pre-stored in a ROM(Read-Only Memory) that is disposed at an appropriate location in thesensor calibration ECU 1, or the like, so that the microcomputerfunctions as functional portions, such as the state detection portion11, the calibratability determination portion 12, the calibrationexecution portion 13, etc., and so that a RAM (Random Access Memory) orthe like disposed at an appropriate location in the sensor calibrationECU 1 functions as the correction coefficient storage portion 14.

The correction coefficient storage portion 14 (which can be regarded asa correction coefficient storage means) is a functional portion thatstores correction coefficients that are used to correct detected valuesfrom the distance image sensor 2, with regard to each of the cellsprovided in the CCD sensor 22 of the distance image sensor 2, in such amanner that the correction coefficients are associated in correspondencewith the brightness information in the correction coefficient storageportion 14. Incidentally, the brightness information and the correctioncoefficient information stored in the correction coefficient storageportion 14 are written in by the calibration execution portion 13, andare read therefrom when the distance image sensor 2 executes thedetection of a distance image. An example of the information stored inthe correction coefficient storage portion 14 will be described laterwith reference to FIG. 6.

The state detection portion 11 (which can be regarded as state detectionmeans) is a functional portion that detects the state of the vehicle VCvia the input appliance 3. Concretely, the state detection portion 11detects the accelerations α1 to α3 in the up-down direction, theleft-right direction and the front-rear direction of the vehicle VC viathe acceleration sensor 31, and detects the yaw angle θ via the yaw ratesensor 32 of the vehicle VC, and detects the vehicle speed V of thevehicle VC via the vehicle speed sensor 33, at every pre-set time (e.g.,10 msec).

The calibratability determination portion 12 (which can be regarded ascalibratability determination means) is a functional portion thatdetermines whether or not the distance image sensor 2 is in acalibratable state on the basis of the state of the vehicle VC and thestate of the road surface SR on which the vehicle VC is located.

Concretely, the calibratability determination portion 12 determineswhether or not the distance image sensor 2 is in the calibratable stateaccording to whether or not the vehicle VC is substantially parallel tothe road surface SR. Besides, the calibratability determination portion12 determines whether or not the distance image sensor 2 is in thecalibratable state, on the basis of the state of the road surface SR.Furthermore, the calibratability determination portion 12 determineswhether or not the distance image sensor 2 is in the calibratable state,on the basis of the state of the vehicle VC detected by the statedetection portion 11.

The method of determining whether or not the vehicle VC is substantiallyparallel to the road surface SR which is performed by thecalibratability determination portion 12 will be described below. Thecalibratability determination portion 12 determines whether or not thevehicle VC is substantially parallel to the road surface SR on the basisof a result of the detection by the distance image sensor 2. If thevehicle VC is determined as being substantially parallel to the roadsurface SR, the calibratability determination portion 12 determines thatthe vehicle VC is in the calibratable state. Concretely, thecalibratability determination portion 12 determines that the vehicle VCis substantially parallel to the road surface SR if the amount of changeΔL in the detected distance during a pre-set time ΔT (e.g., 1 second) isless than or equal to a pre-set threshold value ΔLsh (e.g., 10 mm), withregard to a plurality of pre-set cells among the cells that are providedin the distance image sensor 2.

In this example, the photosensitive cells provided in the distance imagesensor 2 are divided into a plurality of divided regions (e.g., ninedivided regions in a grid arrangement), and the cells each of which ispositioned substantially at the center of a corresponding one of thedivided regions are set as the cells that are used to determine whetheror not the vehicle VC is substantially parallel to the road surface SR.

Thus, on the basis of a result of the detection by the distance imagesensor 2, it is determined whether or not the vehicle VC issubstantially parallel to the road surface SR. According to a result ofthe determination, it is determined whether or not the distance imagesensor 2 is in the calibratable state. Incidentally, although theembodiment will be described in conjunction with the case where thecalibratability determination portion 12 determines whether or not thevehicle VC is substantially parallel to the road surface SR on the basisof a result of the detection by the distance image sensor 2, theinvention is also applicable to a construction in which thecalibratability determination portion 12 determines whether or not thevehicle VC is substantially parallel to the road surface SR on the basisof the detection result provided by a sensor other than the distanceimage sensor 2 (e.g., the yaw rate sensor 32 or the like), instead of(or in addition to) the detection result provided by the distance imagesensor 2.

Besides, it is determined whether or not the vehicle VC is substantiallyparallel to the road surface SR on the basis of the detection resultprovided by the distance image sensor 2, and according to the result ofthe determination, it is determined whether or not the distance imagesensor 2 is in the calibratable state, as described above. Therefore,the distance image sensor 2 can be appropriately calibrated.

That is, in the case where the vehicle VC is substantially parallel tothe road surface SR, the distance image sensor 2 can be properlycalibrated by using the distance that is found beforehand on thesupposition that the vehicle VC is parallel to a flat road surface SR0(distances L1 to L3 between the distance image sensor 2 and the roadsurface SR0 (see FIG. 4)).

Furthermore, it is determined that the vehicle VC is substantiallyparallel to the road surface SR if the amount of change ΔL in thedetected distance during the pre-set time ΔT is less than or equal tothe pre-set threshold value ΔLsh, with regard to each of the cellscontained in the CCD sensor 22 of the distance image sensor 2.Therefore, it can be properly determined that the vehicle VC issubstantially parallel to the road surface SR.

That is, when the amount of change ΔL during the pre-set time ΔT in thedistance detected by each of the cells provided in the CCD sensor 22 ofthe distance image sensor 2 is “0”, the vehicle VC is moving parallel tothe road surface SR during that period of time. Therefore, when theamount of change ΔL is less than or equal to the pre-set threshold valueΔLsh, it can be determined that the vehicle VC is substantially parallelto the road surface SR.

In addition, since it is determined that the vehicle VC is substantiallyparallel to the road surface SR in the case where the amount of changeΔL in the detected distance during the pre-set time ΔT is less than orequal to the pre-set threshold value ΔLsh, with regard to a plurality ofpre-set cells among the cells provided in the CCD sensor 22 of thedistance image sensor 2, it is possible to easily determine that thevehicle VC is substantially parallel to the road surface SR.

That is, since it suffices to determine whether or not the amount ofchange ΔL in the detected distance during the pre-set time ΔT is lessthan or equal to the pre-set threshold value ΔLsh with regard to aplurality of pre-set cells among the cells that are provided in the CCDsensor 22 of the distance image sensor 2, the process needed for thedetermination is simplified.

Next, a method of determining whether or not the distance image sensor 2is in the calibratable state on the basis of the state of the roadsurface SR which is performed by the calibratability determinationportion 12 will be described. The calibratability determination portion12 finds a variance σ of the brightnesses detected by the cellscontained in the CCD sensor 22 of the distance image sensor 2, anddetermines whether or not the distance image sensor 2 is in thecalibratable state according to whether or not the variance σ is lessthan or equal to a pre-set threshold value ash.

Concretely, each of the cells contained in the CCD sensor 22 of thedistance image sensor 2 is set in a corresponding one of the pre-setdivided regions (nine regions in this embodiment). Then, thecalibratability determination portion 12 finds a variance σ of thebrightnesses detected by the cells contained in each of the dividedregions, and determines whether or not the divided region of thedistance image sensor 2 is in the calibratable state according towhether or not the variance σ in the divided region is less than orequal to the pre-set threshold value σsh, with regard to each of thedivided regions.

FIGS. 3A to 3C are illustrative diagrams showing examples of thedistribution of brightness on a road surface SR that are detected by theCCD sensor 22 of the distance image sensor 2. The road surface SRdetected by the CCD sensor 22 of the distance image sensor 2 has atrapezoidal shape as shown in FIGS. 3A to 3C. FIG. 3A is a brightnessdistribution diagram YD1 regarding the road surface SR in the case wherethe brightness is uniform over the surface. In the case where thebrightness is uniform, it is determined by the calibratabilitydetermination portion 12 that the distance image sensor 2 is in thecalibratable state.

FIG. 3B is a brightness distribution diagram YD2 regarding the roadsurface SR in the case where the brightness is non-uniform over thesurface. In the brightness distribution diagram YD2, there are a regionBA with direct sun shine, and a region DA that has a water puddle, andtherefore has a reflectance that is lower than that of the road surfaceSR. In this region BA, since the backlight is strong, the shot noiseincreases, so that the S/N ratio, which is a factor of distance error ofthe distance image sensor 2, declines and therefore proper measurementof distance cannot be performed. In the region DA with low brightness,since the reflectance is low, reflected light of the light emitted fromthe LED 21 is scarcely input to the CCD sensor 22; so that the S/N ratioof the distance image sensor 2 declines and therefore proper distancemeasurement cannot be performed.

FIG. 3C is a brightness distribution diagram YD2 of the road surface SRin the case where the state of the road surface is the same as the stateshown in FIG. 3B, and where the CCD sensor 22 is divided into ninedivided regions. As shown in FIG. 3C, the brightness distributiondiagram YD2 is divided into nine divided regions YD21 to YD29. Then,since the divided regions YD24 to YD26 contain the low-brightness regionDA, the calibratability determination portion 12 determines that thedivided regions YD24 to YD26 are not in the calibratable state. Besides,since the divided region YD29 contains the region BA that has strongbacklight, the calibratability determination portion 12 determines thatthe divided region YD29 is not in the calibratable state. On the otherhand, since the divided regions YD21 to YD23, YD27 and YD28 are regionsof the road surface that are in an environment with relatively weakbackground light and that have a certain reflectance or higher (do nothave a water puddle or the like), the calibratability determinationportion 12 determines that the divided regions YD21 to YD23, YD27 andYD28 are in the calibratable state.

Thus, of the plurality of (nine in this embodiment) divided regions YD21to YD29, the divided regions (in this embodiment, the divided regionsYD21 to YD23, YD27 and YD28) in which the road surface SR has asubstantially uniform brightness distribution (i.e., in which thevariance σ of the brightness is less than or equal to the pre-setthreshold value ash) are determined as being in the calibratable stateby the calibratability determination portion 12. Therefore, it can beproperly determined whether or not a divided region is in thecalibratable state with regard to each of the divided regions YD21 toYD29, so that the calibration accuracy can be improved. Besides, even inthe case where the calibration cannot be carried out for all the cellscontained in the CCD sensor 22 of the distance image sensor 2 (i.e., thecase where the distance image sensor 2 as a whole is not in thecalibratable state), the calibration can be performed separately foreach of the divided regions YD21 to YD29 of the CCD sensor 22 of thedistance image sensor, so that it is possible to efficiently calibratethe distance image sensor 2.

Although this embodiment will be described in conjunction with the casewhere each of the cells contained in the CCD sensor 22 of the distanceimage sensor 2 is set in a corresponding one of the nine divisionsregion YD21 to YD29 as shown in FIG. 3C, it suffices that each of thecells contained in the CCD sensor 22 of the distance image sensor 2 isset in a corresponding one of a plurality of divided regions. That is,the number of divided regions may also be a number other than nine. Thegreater the number of divided region, the higher the possibility ofperformance of the calibration of a divided region can be made.Conversely, the smaller the number of divided regions, the simpler theprocess can be made, and the more the accuracy of the calibration can beimproved. Besides, in this embodiment, it is possible to determine thatthe distance calibration be started if, as a calibration start conditionfor the distance image sensor 2, the brightness information regardingthe picture-taking area (a road surface, a wall, etc.) indicates thatthe brightness (S/N ratio) is higher than or equal to a certainbrightness and the variance of the entire brightness distribution(histogram) is within a certain range.

Referring back to FIG. 1 again, a functional construction of the sensorcalibration ECU 1 will be described. The calibratability determinationportion 12 is also a functional portion that determines whether or notthe distance image sensor 2 is in the calibratable state on the basis ofa result of the detection by the state detection portion 11. Concretely,the calibratability determination portion 12 determines whether or notthe distance image sensor 2 is in the calibratable state on the basis ofthe accelerations α1 to α3 detected via the acceleration sensor 31, theyaw angle θ detected by the yaw rate sensor 32, and the vehicle speed Vdetected via the vehicle speed sensor 33.

That is, the calibratability determination portion 12 determines thatthe distance image sensor 2 is in the calibratable state if thefollowing three conditions (conditions A to C) are satisfied. ConditionA: the accelerations α1 to α3 are each less than or equal to theirrespective pre-set threshold values. Condition B: the yaw angle θ isless than or equal to a pres-set threshold value. Condition C: thevehicle speed V is less than or equal to a pres-set threshold value.

The state of the vehicle VC is detected as described above, and then itis determined, on the basis of a result of the detection, whether or notthe distance image sensor 2 is in the calibratable state. Therefore, itcan be properly determined whether or not the distance image sensor 2 isin the calibratable state.

In the case where the accelerations α1 to α3 detected by theacceleration sensor 31 are small, it can be estimated that the travelingstate of the vehicle VC is stable, so that it can be determined that thedistance image sensor 2 is in the calibratable state. In the case wherethe yaw angle θ detected by the yaw rate sensor 32 is large, it can beestimated that the vehicle VC is not substantially parallel to the roadsurface, so that it can be determined that the distance image sensor 2is not in the calibratable state. In addition, in the case where thevehicle speed V detected by the vehicle speed sensor 33 is small, it canbe estimated that the change in the traveling state of the vehicle VC issmall, so that it can be determined that the distance image sensor 2 isin the calibratable state.

This embodiment is described in conjunction with the case where thecalibratability determination portion 12 determines whether or not thedistance image sensor 2 is in the calibratable state on the basis of theaccelerations α1 to α3, the yaw angle θ, and the vehicle speed V.However, the invention is also applicable to a construction in which thecalibratability determination portion 12 determines whether or not thedistance image sensor 2 is in the calibratable state on the basis of adetection value that indicates a state of the vehicle VC instead of (orin addition to) the accelerations α1 to α3, the yaw angle θ and thevehicle speed V. For example, the calibratability determination portion12 may also determine whether or not the distance image sensor 2 is inthe calibratable state on the basis of the pitch angle and the rollangle of the vehicle, instead of (or in addition to) the yaw angle θ.

Referring back to FIG. 1 again, a functional construction of the sensorcalibration ECU 1 will be described. In the case where thecalibratability determination portion 12 has determined that thedistance image sensor 2 is in the calibratable state, the calibrationexecution portion 13 (that corresponds to calibration execution means)calibrates the distance image sensor 2 on the basis of a pre-founddistance between the distance image sensor 2 and the road surface SR onwhich the vehicle VC is positioned (hereinafter, referred to as“reference distance”).

FIG. 4 is a side view illustrating an example of the reference distancethat is a distance that serves as a reference for the calibration. Thereference distance is set, for example, as shown in FIG. 4 in the casewhere the road surface SR is a flat surface parallel to the vehicle VC(not shown) (the road surface SR in this case will hereinafter bereferred to as “reference road surface SR0”). That is, the referencedistance is set as an optical path length (e.g., optical path lengthsL1, L2, L3, etc.) along which the light ray emitted from the LED 21 ofthe distance image sensor 2 is reflected by the reference road surfaceSR0, and then reaches the CCD sensor 22. In addition, optical pathlengths L1, L2, L3, etc., with respect to the distance L0 between theLED 21 and a calibration reference point of the CCD sensor 22 are knownvalues.

That is, the reference distance (=optical path length L1, L2, L3, etc.)can be calculated beforehand according to a geometric relation of thedistance image sensor 2 (the LED 21 and the CCD sensor 22) with thereference road surface SR0 which is determined on the basis of theposition of the distance image sensor 2 in the vehicle VC. Therefore,the distance image sensor 2 can easily be calibrated on the basis of thepre-calculated reference distance.

Besides, the calibration execution portion 13 performs the calibrationfor each of a plurality of brightnesses γn1 and γn2 (two brightnesses inthis embodiment), and records correction coefficients found as resultsof the calibration (that are a slope An1, a y-axis intercept Bn1, aslope An2, and a y-axis intercept Bn2, as shown in FIGS. 5A and 5B, andFIG. 6, in this embodiment) in the correction coefficient storageportion 14 in such a manner that the correction coefficients areassociated in correspondence with the brightnesses γn1 and γn2. It is tobe noted herein that the cell ID No. n is a number for distinguishingthe photosensitive cells that constitute the CCD sensor 22, and is oneof 1 to M (where M is the number of the cells of the CCD sensor 22).Incidentally, the brightness γn2 is greater than the brightness γn1.

FIGS. 5A and 5B are graphs showing examples of the correctioncoefficient-calculating methods performed by the calibration executionportion 13. In the graphs, the horizontal axis represents a detecteddistance X, and the vertical axis represents a corrected distance Y.Incidentally, the correction distance Y is a distance that is to bedetected (=true value of the distance), and is the reference distance(=optical path length L1, L2, L3, etc.) mentioned above with referenceto FIG. 4. FIG. 5A shows a graph G1 showing an example of the method ofcalculating the correction coefficients (the slope An1, and theintercept Bn1 in this embodiment) at the brightness γn1, and FIG. 5Bshows a graph G2 showing an example of the method of calculating thecorrection coefficients (the slope An2, and the intercept Bn2 in thisembodiment) at the brightness γn2.

A measurement point P10 in FIG. 5A is a measurement point in the casewhere light projected from the LED 21 enters a cell of cell ID No. n inthe CCD sensor 22 directly (i.e., without being reflected by the roadsurface SR or the like). Another measurement point P11 is a measurementpoint in the case where light projected from the LED 21 is reflected bythe road surface SR, and the reflected light therefrom enters the cellof cell ID No. n in the CCD sensor 22. The graph G1 is a straight linepassing through the measurement point P10 and the measurement point P11,and is expressed by the following equation (1).Y=An1×X+Bn1  (1)

That is, the calibration execution portion 13 acquires a detecteddistance (L0α/2) and a detected distance (Lnα) that correspond to thetwo different measurement points, that is, the measurement point P10 andthe measurement point P11, respectively, via the distance image sensor2, and compares the detected distances with the reference distances(i.e., corrected distances) (L0/2) and (Ln) that correspond to themeasurement point P10 and the measurement point P11, respectively, so asto find correction coefficients (the slope An1, and the intercept Bn1 inthis embodiment).

Similarly, a measurement point P20 in FIG. 5B is a measurement point inthe case where light projected from the LED 21 enters cell of cell IDNo. n in the CCD sensor 22 directly (i.e., without being reflected bythe road surface SR or the like). Another measurement point P21 is ameasurement point in the case where light projected from the LED 21 isreflected by the road surface SR, and the reflected light therefromenters the cell of cell ID No. n in the CCD sensor 22. The graph G2 is astraight line passing through the measurement point P20 and themeasurement point P21, and is expressed by the following equation (2).Y=An2×X+Bn2  (2)

That is, the calibration execution portion 13 acquires a detecteddistance (L0β/2) and a detected distance (Lnβ that correspond to the twodifferent measurement points, that is, the measurement point P20 and themeasurement point P21, respectively, via the distance image sensor 2,and compares the detected distances with the reference distances (i.e.,corrected distances) (L0/2) and (Ln) that correspond to the measurementpoint P20 and the measurement point P21, respectively, so as to findcorrection coefficients (the slope An2, and the intercept Bn2 in thisembodiment).

FIG. 6 is a table showing examples of the correction coefficients storedin the correction coefficient storage portion 14. In the table, theextreme left column shows the cell ID Nos. n. For each cell ID No. n,the slope A and the intercept B associated in correspondence with thebrightness γ are stored as low-brightness-side correction coefficients,and, similarly, the slope A and the intercept B associated incorrespondence with the brightness γ are stored as high-brightness-sidecorrection coefficients.

FIGS. 7A and 7B show graphs showing examples of methods of correcting adetected value from the distance image sensor 2 by using correctioncoefficients stored in the correction coefficient storage portion 14.FIG. 7A shows a graph G3 showing an example of the method of finding theslope. An for use in the correction, and FIG. 7B shows a graph G3showing an example of the method of finding the intercept Bn for use inthe correction. It is to be noted herein that the cell ID No. n is anumber for identifying the photosensitive cells that constitute the CCDsensor 22, and is one of 1 to M (M is the number of the cells of the CCDsensor 22). In FIG. 7A, the horizontal axis represents the brightness γ,and the vertical axis represents the slope A. In FIG. 7B, the horizontalaxis represents the brightness γ, and the vertical axis represents theintercept B.

In FIG. 7A, the correction point P30 is a point that corresponds to alow-brightness-side correction coefficient, and the correction point P31is a point that corresponds to a high-brightness-side correctioncoefficient. The graph G3 is a straight line passing through thecorrection point P30 and the correction point P31. In the case where thebrightness detected by cells that correspond to cell ID Nos. n is abrightness γn, a correction coefficient (=slope An) for use in thecorrection is found through the use of the graph G3 as shown in FIG. 7A.

In FIG. 7B, a correction point P40 is a point that corresponds to alow-brightness-side correction coefficient, and a correction point P41is a point that corresponds to a high-brightness-side correctioncoefficient. The graph G4 is a straight line passing through thecorrection point P40 and the correction point P41. In the case where thebrightness detected by cells that correspond to cell ID Nos. n is abrightness γn, a correction coefficient (=intercept Bn) for use in thecorrection is found through the use of the graph G4 as shown in FIG. 7B.

Then, using the slope An found through the use of the graph shown inFIG. 7A as described above, and the intercept Bn found through the useof the graph shown in FIG. 7B as described above, a corrected distance Yis found from the detected distance X through the following equation(3).Y=An×X+Bn  (3)

Thus, the calibration is performed for each of a plurality ofbrightnesses γn1 and γn2 (two brightnesses in this embodiment), andcorrection coefficients found as results of the calibration (that arethe slope An1, the y-axis intercept Bn1, the slope An2, and the y-axisintercept Bn2 in this embodiment) are recorded in the correctioncoefficient storage portion 14 in such a manner that the correctioncoefficients are associated in correspondence with the brightnesses γn1and γn2. Therefore, since proper correction coefficients (the slope An,and the intercept Bn) can be found according to the brightness γn at thetime of detection, through the use of a plurality of correctioncoefficients (two correction coefficients in this embodiment) thatcorrespond to a plurality of brightnesses (two brightnesses in thisembodiment) at the time of calibration, it is possible to perform aproper calibration. For example, in the case where two correctioncoefficients (the slope An1, the intercept Bn1, the slope An2, and theintercept Bn2 in this embodiment) that correspond to each of the twobrightnesses γn1 and γn2 at the time of calibration stored in thecorrection coefficient storage portion 14, proper correctioncoefficients (the slope An and the intercept Bn in this embodiment) canbe found through linear interpolation of the two correction coefficientsaccording to the brightnesses γn at the time of detection. As describedabove, in this embodiment, the results of the distance correction foreach of the brightnesses can be obtained with respect to the detectedobjects (each pixel). Then, by further implementing the linearinterpolation on the basis of the detected brightness information, itbecomes possible to perform a distance calibration process that takesinto account the brightness as well.

Although the embodiment has been described in conjunction with the casewhere the calibration execution portion 13 calibrates the distance imagesensor 2 by linear interpolation for each of the two brightnesses γn1and γn2, the invention is also applicable to a construction in which thecalibration execution portion 13 performs the calibration by curveinterpolation for each of the three or more brightnesses. In such acase, more proper correction coefficients can be found. Besides, it isalso permissible to adopt a construction in which the calibrationexecution portion 13 performs the calibration by differenceinterpolation at one brightness.

FIG. 8 is a flowchart showing an example of the operation of the sensorcalibration ECU 1. Firstly, the calibratability determination portion 12executes a calibratability determination process that is a process ofdetermining whether or not the distance image sensor 2 is in thecalibratable state (S101). Then, the calibration execution portion 13determines whether or not there exists a region in which the calibrationis feasible. If it is determined that there is no region in which thecalibration is feasible (NO in S103), the process returns to step S101to repeat the process starting in step S101. If it is determined thatthere is a region in which the calibration is feasible (YES in S103),the calibration execution portion 13 selects a photosensitive cell thatis contained in the calibration-feasible region (S105).

Then, for each of the cells selected in step S105, a detected distanceL0α (or L0β) in the case where light projected from the LED 21 enters acell of cell ID No. n in the CCD sensor 22 directly (i.e., without beingreflected by the road surface SR or the like) is detected (S107). Next,for each of the cells selected in step S105, a detected distance Lnα (orLnβ) in the case where light projected from the LED 21 is reflected bythe road surface SR, and the reflected light therefrom enters the cellof cell ID No. n in the CCD sensor 22 is detected (S109). Subsequently,the calibratability determination portion 12 executes thecalibratability determination process again with respect to the regionthat is determined as being a calibration-feasible region in step S101(S111).

Then, the calibration execution portion 13 determines whether or notthere is a region in which the calibration is feasible (S113). If it isdetermined that there is no calibration-feasible region (NO in S113),the process returns to step S101 to repeat the process starting in stepS101. If it is determined that there is a calibration-feasible region(YES in S113), correction coefficients (the slope An1 and the interceptBn1, or the slope An2 and the intercept Bn2) are found (S115 and S117)by comparing the detected distance L0α (or L0β) detected in step S107and the detected distance Lnα (or Lnβ) detected in step S109 with thereference distances (=corrected distances) (L0/2) and (Ln). Next, by thecalibration execution portion 13, the correction coefficients (the slopeAn1 and the intercept Bn1, or the slope An2 and the intercept Bn2) foundin steps S115 and S117 are recorded in the correction coefficientstorage portion 14 (S119) in such a manner that the correctioncoefficients are associated in correspondence with the brightness γn1(the brightness γn2).

Incidentally, step S101 and step S111 in the flowchart shown in FIG. 8can be regarded as a calibratability determination step in the sensorcalibration method in accordance with the invention, and steps S103 to109 and steps S113 to S119 in the flowchart shown in FIG. 8 can beregarded as a calibration execution step in the sensor calibrationmethod in accordance with the invention.

FIG. 9 is a detailed flowchart showing an example of the calibratabilitydetermination process that is executed in step S101 and step S111 in theflowchart shown in FIG. 8. For the sake of convenience, the followingdescription will be made in conjunction with the case where theaccelerations α1 to α3, the yaw angle θ and the vehicle speed V havebeen detected beforehand by the state detection portion 11.Incidentally, the following process is all performed by thecalibratability determination portion 12. Firstly, it is determinedwhether or not all the accelerations α1 to α3 are less than or equal toa pre-set threshold value (S201). If it is determined that any one ofthe accelerations α1 to α3 is larger than the threshold value (NO inS201), the process proceeds to step S207.

If all the accelerations α1 to α3 are less than or equal to thethreshold value (YES in S201), it is then determined whether or not theyaw angle θ is less than or equal to a pre-set threshold value (S203).If it is determined that the yaw angle θ is larger than the thresholdvalue (NO in S203), the process proceeds to step S207. If it isdetermined that the yaw angle θ is less than or equal to the thresholdvalue (YES in S203), it is then determined whether or not the vehiclespeed V is less than or equal to a pre-set threshold value (S205). If itis determined that the vehicle speed V is larger than the thresholdvalue (NO in S205), the process proceeds to step S207.

If NO is the answer to the determination in step S201, or if NO in stepS203, or if NO in step S205, or if NO in step S211, it is thendetermined that the distance image sensor 2 is not in the calibratablestate (S207), and the process returns to step S103 (or step S113) shownin FIG. 8.

If it is determined that the vehicle speed V is less than or equal tothe threshold value (YES in S205), an amount of change ΔL in thedetected distance is calculated for each of the cells each of which ispositioned substantially at the center of a corresponding one of thenine divided regions (see FIG. 3C) (S209). Then, with regard to each ofthe substantially center cells of the nine divided regions (see FIG.3C), it is determined whether or not the amount of change ΔL in thedetected distance calculated in step S209 is less than or equal to athreshold value ΔLsh (S211). If it is determined that at least one ofthe amounts of change ΔL that correspond one-to-one to the dividedregions is larger than the threshold value ΔLsh (NO in S211), theprocess proceeds to step S207. If all the amounts of change ΔLcorresponding one-to-one to the divided regions (nine divided regions inthis embodiment) are less than or equal to the threshold value ΔLsh (YESin S211), the brightness γn is detected via each of the correspondingcells in the CCD sensor 22 (S213). Then, for each of the nine dividedregions (see FIG. 3), the variance σ of the brightness γn detected bythe cells contained in the divided region is calculated (S215). Next,for each of the divided regions, it is determined whether or not thevariance σ is less than or equal to a pre-set threshold value ash(S217).

If it is determined that the variance σ of a divided region is less thanor equal to the threshold value ash (YES in S217), it is then determinedthat the divided region is in the calibratable state (S219), and theprocess returns to step S103 (or step S113) shown in FIG. 8. If it isdetermined that the variance σ of a divided region is larger than thethreshold value ash (NO in S217), it is then determined that the dividedregion is not in the calibratable state (S221), and the process returnsto step S103 (or step S113) shown in FIG. 8.

Since it is determined whether or not the distance image sensor 2 is inthe calibratable state on the basis of the state of the vehicle VC andthe state of the road surface SR on which the vehicle VC is positionedas described above, it can be properly determined whether or not thedistance image sensor 2 is in the calibratable state. Furthermore, sincethe distance image sensor 2 is calibrated on the pre-found distance(=reference distance) between the distance image sensor 2 and the roadsurface SR on which the vehicle VC is positioned, it is possible toeasily calibrate the distance image sensor 2.

Although the embodiment has been described in conjunction with the casewhere the calibratability determination portion 12 determines whether ornot the distance image sensor 2 is in the calibratable state on thebasis of the state of the vehicle VC and the state of the road surfaceSR on which the vehicle VC is positioned, it suffices that aconstruction is provided in which the calibratability determinationportion 12 determines whether or not the distance image sensor 2 is inthe calibratable state on the basis of at least one of the state of thevehicle VC and the state of the road surface SR on which the vehicle VCis positioned.

Incidentally, the sensor calibration device in accordance with theinvention is not limited to the foregoing embodiments, but may also beconstructed as follows. (A) Although the embodiment has been describedin conjunction with the case where the sensor calibration ECU 1functionally includes the state detection portion 11, thecalibratability determination portion 12, the calibration executionportion 13, etc., it is also permissible to adopt a construction inwhich at least one of these functional portions, that is, the statedetection portion 11, the calibratability determination portion 12, andthe calibration execution portion 13, is constructed by a hardwaredevice such as an electric circuit or the like.

(B) Although the embodiment has been described in conjunction with thecase where the calibratability determination portion 12 performs thecalibratability determination process before and after acquiring datafor use in the calibration (i.e., performs the process in step S101 andstep S111) as shown in the flowchart of FIG. 8, it is also permissibleto adopt a construction in which the calibratability determinationportion 12 performs the calibratability determination process at leastone of before and after acquiring data for use in the calibration. Inthis case, the process is simplified.

This invention is applicable to, for example, a sensor calibrationdevice that calibrates a distance-measuring sensor that is mounted in avehicle, and to a sensor calibration method for such adistance-measuring sensor. In particular, the invention is applicable toa sensor calibration device that calibrates a distance image sensor thatis mounted in a vehicle, and to a sensor calibration method for such adistance image sensor.

What is claimed is:
 1. A sensor calibration device that calibrates adistance-measuring sensor that is mounted in a vehicle, comprising: acalibratability determination portion that determines whether or not thedistance-measuring sensor is in a calibratable state based on at leastone of state of the vehicle and state of a road surface on which thevehicle is positioned; and a calibration execution portion thatdetermines a correction coefficient to calibrate the distance-measuringsensor when the distance-measuring sensor is determined to be in thecalibratable state, the correction coefficients being determined basedon a pre-found distance between the distance-measuring sensor and theroad surface on which the vehicle is positioned, the calibrationexecution portion mathematically applying the correction coefficient toa detected value detected by the distance-measuring sensor to generate acorrected value, wherein the distance-measuring sensor is a distanceimage sensor at least a portion of whose detection region is the roadsurface, and the calibratability determination portion determineswhether or not the distance image sensor is in the calibratable statebased on a result of detection by the distance image sensor, thecalibratability determination portion determines whether or not thevehicle is substantially parallel to the road surface based on theresult of detection by the distance image sensor, and then determinesthat the distance image sensor is in the calibratable state when thevehicle is substantially parallel to the road surface, and thecalibratability determination portion determines that the vehicle issubstantially parallel to the road surface when, regarding a pluralityof pre-set photosensitive cells among photosensitive cells contained inthe distance image sensor, an amount of change in distance detected byeach of the pre-set photosensitive cells during a pre-set time is lessthan or equal to a pre-set threshold value.
 2. A sensor calibrationdevice that calibrates a distance-measuring sensor that is mounted in avehicle, comprising: a calibratability determination portion thatdetermines whether or not the distance-measuring sensor is in acalibratable state based on at least one of state of the vehicle andstate of a road surface on which the vehicle is positioned; and acalibration execution portion that determines a correction coefficientto calibrate the distance-measuring sensor when the distance-measuringsensor is determined to be in the calibratable state, the correctioncoefficients being determined based on a pre-found distance between thedistance-measuring sensor and the road surface on which the vehicle ispositioned, the calibration execution portion mathematically applyingthe correction coefficient to a detected value detected by thedistance-measuring sensor to generate a corrected value, wherein: thedistance-measuring sensor is a distance image sensor at least a portionof whose detection region is the road surface, the calibratabilitydetermination portion determines whether or not the distance imagesensor is in the calibratable state based on a result of detection bythe distance image sensor, and the calibratability determination portionfinds a variance of brightnesses detected by photosensitive cells thatare contained in the distance image sensor, and determines whether ornot the distance image sensor is in the calibratable state according towhether or not the variance is less than or equal to a pre-set thresholdvalue.
 3. A sensor calibration device that calibrates adistance-measuring sensor that is mounted in a vehicle, comprising: acalibratability determination portion that determines whether or not thedistance-measuring sensor is in a calibratable state based on at leastone of state of the vehicle and state of a road surface on which thevehicle is positioned; and a calibration execution portion thatdetermines a correction coefficient to calibrate the distance-measuringsensor when the distance-measuring sensor is determined to be in thecalibratable state, the correction coefficients being determined basedon a pre-found distance between the distance-measuring sensor and theroad surface on which the vehicle is positioned, the calibrationexecution portion mathematically applying the correction coefficient toa detected value detected by the distance-measuring sensor to generate acorrected value, wherein: the distance-measuring sensor is a distanceimage sensor at least a portion of whose detection region is the roadsurface, the calibratability determination portion determines whether ornot the distance image sensor is in the calibratable state based on aresult of detection by the distance image sensor, each of photosensitivecells contained in the distance image sensor is set in a correspondingone of a plurality of pre-set divided regions, the calibratabilitydetermination portion, with regard to each of the divided regions, findsa variance of brightnesses detected by the photosensitive cellscontained in a divided region, and determines whether or not the dividedregion of the distance image sensor is in the calibratable stateaccording to whether or not the variance is less than or equal to apre-set threshold value, and the calibration execution portioncalibrates the divided region of the distance image sensor that isdetermined as being in the calibratable state by the calibratabilitydetermination portion.
 4. The sensor calibration device according toclaim 1, further comprising: a correction coefficient storage portionthat stores the correction coefficient for use for correcting a detectedvalue from the distance image sensor, with regard to each ofphotosensitive cells contained in the distance image sensor, wherein thecalibration execution portion calibrates the distance image sensor withregard to a plurality of brightnesses, and records in the correctioncoefficient storage portion a correction coefficient found as a resultof calibration so that the correction coefficient is associated incorrespondence with information about the brightness.
 5. The sensorcalibration device according to claim 1, further comprising: a statedetection portion that detects state of the vehicle, wherein thecalibratability determination portion determines whether or not thedistance-measuring sensor is in the calibratable state based on a resultof detection by the state detection portion.
 6. The sensor calibrationdevice according to claim 5, wherein the state detection portionincludes at least one of an acceleration sensor that detectsacceleration of the vehicle, an inclination detection sensor thatdetects inclination of the vehicle, and a vehicle speed sensor thatdetects vehicle speed of the vehicle.
 7. The sensor calibration deviceaccording to claim 6, wherein the inclination detection sensor detectsat least one of yaw angle, pitch angle and roll angle of the vehicle. 8.A sensor calibration method that calibrates a distance-measuring sensorthat is mounted in a vehicle, comprising: determining whether or not thedistance-measuring sensor is in a calibratable state based on at leastone of state of the vehicle and state of a road surface on which thevehicle is positioned; and calibrating the distance-measuring sensor bydetermining correction coefficients to be mathematically applied todetected values detected by the distance-measuring sensor to generatecorrected values, the correction coefficients being based on a pre-founddistance between the distance-measuring sensor and the road surface onwhich the vehicle is positioned, when the distance-measuring sensor isdetermined to be in the calibratable state, wherein thedistance-measuring sensor is a distance image sensor at least a portionof whose detection region is the road surface, and whether or not thedistance image sensor is in the calibratable state is determined basedon a result of detection by the distance image sensor, whether or notthe vehicle is substantially parallel to the road surface is determinedbased on the result of detection by the distance image sensor, and thedistance image sensor is determined to be in the calibratable state whenthe vehicle is substantially parallel to the road surface, and thevehicle is determined to be substantially parallel to the road surfacewhen, regarding a plurality of pre-set photosensitive cells amongphotosensitive cells contained in the distance imaging sensor, an amountof change in distance detected by each of the pre-set photosensitivecells during a pre-set time is less than or equal to a pre-set thresholdvalue.
 9. The sensor calibration device according to claim 4, whereinthe correction coefficient storage portion stores correctioncoefficients corresponding to at least two brightness levels for each ofthe photosensitive cells contained in the distance image sensor.
 10. Thesensor calibration device according to claim 9, wherein the correctioncoefficients include a slope and an intercept of a graph used to correctthe detected value from the distance image sensor.
 11. The sensorcalibration device according to claim 6, wherein the accelerationdetector detects vertical acceleration, a first horizontal accelerationand a second horizontal acceleration perpendicular to the firsthorizontal acceleration.
 12. The calibration device according to claim1, wherein the distance measuring sensor is a charged coupled device.13. The calibration device according to claim 1, further comprising: alight source that illuminates the road surface, wherein the image sensorreceives light reflected by the road surface, and the calibratabilitydetermination portion determines whether or not the distance imagesensor is in the calibratable state based on detection of the reflectedlight.
 14. The calibration device according to claim 13, wherein thecalibratability determination portion determines whether or not thedistance image sensor is in the calibratable state by measuring a timeof flight of light from the light source to the image sensor viareflection on the road surface.
 15. The calibration device according toclaim 13, wherein the light source is a light emitting diode.